ei

routines for handling the erlang binary term format

The library ei contains macros and functions to encode
and decode the erlang binary term format.

With ei, you can convert atoms, lists, numbers and
binaries to and from the binary format. This is useful when
writing port programs and drivers. ei uses a given
buffer, and no dynamic memory (with the exception of
ei_decode_fun()), and is often quite fast.

It also handles C-nodes, C-programs that talks erlang
distribution with erlang nodes (or other C-nodes) using the
erlang distribution format. The difference between ei and
erl_interface is that ei uses the binary format
directly when sending and receiving terms. It is also thread
safe, and using threads, one process can handle multiple
C-nodes. The erl_interface library is built on top of
ei, but of legacy reasons, it doesn't allow for multiple
C-nodes. In general, ei is the preferred way of doing
C-nodes.

The decode and encode functions use a buffer an index into the
buffer, which points at the point where to encode and
decode. The index is updated to point right after the term
encoded/decoded. No checking is done whether the term fits in
the buffer or not. If encoding goes outside the buffer, the
program may crash.

All functions takes two parameter, buf is a pointer to
the buffer where the binary data is / will be, index is a
pointer to an index into the buffer. This parameter will be
incremented with the size of the term decoded / encoded. The
data is thus at buf[*index] when an ei function is
called.

The encode functions all assumes that the buf and
index parameters points to a buffer big enough for the
data. To get the size of an encoded term, without encoding it,
pass NULL instead of a buffer pointer. The index
parameter will be incremented, but nothing will be encoded. This
is the way in ei to "preflight" term encoding.

There are also encode-functions that uses a dynamic buffer. It
is often more convenient to use these to encode data. All encode
functions comes in two versions: those starting with ei_x,
uses a dynamic buffer.

All functions return 0 if successful, and -1 if
not. (For instance, if a term is not of the expected type, or
the data to decode is not a valid erlang term.)

Some of the decode-functions needs a preallocated buffer. This
buffer must be allocated big enough, and for non compound types
the ei_get_type()
function returns the size required (note that for strings an
extra byte is needed for the 0 string terminator).

By default, the ei library is only guaranteed
to be compatible with other Erlang/OTP components from the same
release as the ei library itself. For example, ei from
the OTP R10 release is not compatible with an Erlang emulator
from the OTP R9 release by default.

A call to ei_set_compat_rel(release_number) sets the
ei library in compatibility mode of release
release_number. Valid range of release_number
is [7, current release]. This makes it possible to
communicate with Erlang/OTP components from earlier releases.

Note!

If this function is called, it may only be called once
and must be called before any other functions in the ei
library is called.

Warning!

You may run into trouble if this feature is used
carelessly. Always make sure that all communicating
components are either from the same Erlang/OTP release, or
from release X and release Y where all components
from release Y are in compatibility mode of release X.

int ei_encode_version(char *buf, int *index)

int ei_x_encode_version(ei_x_buff* x)

Encodes a version magic number for the binary format. Must
be the first token in a binary term.

int ei_encode_long(char *buf, int *index, long p)

int ei_x_encode_long(ei_x_buff* x, long p)

Encodes a long integer in the binary format.
Note that if the code is 64 bits the function ei_encode_long() is
exactly the same as ei_encode_longlong().

int ei_encode_ulong(char *buf, int *index, unsigned long p)

int ei_x_encode_ulong(ei_x_buff* x, unsigned long p)

Encodes an unsigned long integer in the binary format.
Note that if the code is 64 bits the function ei_encode_ulong() is
exactly the same as ei_encode_ulonglong().

int ei_encode_longlong(char *buf, int *index, long long p)

int ei_x_encode_longlong(ei_x_buff* x, long long p)

Encodes a GCC long long or Visual C++ __int64 (64 bit)
integer in the binary format. Note that this function is missing
in the VxWorks port.

int ei_encode_ulonglong(char *buf, int *index, unsigned long long p)

int ei_x_encode_ulonglong(ei_x_buff* x, unsigned long long p)

Encodes a GCC unsigned long long or Visual C++ unsigned __int64 (64 bit) integer in the binary format. Note that
this function is missing in the VxWorks port.

int ei_encode_bignum(char *buf, int *index, mpz_t obj)

int ei_x_encode_bignum(ei_x_buff *x, mpz_t obj)

Encodes a GMP mpz_t integer to binary format.
To use this function the ei library needs to be configured and compiled
to use the GMP library.

int ei_encode_double(char *buf, int *index, double p)

int ei_x_encode_double(ei_x_buff* x, double p)

Encodes a double-precision (64 bit) floating point number in
the binary format.

int ei_encode_boolean(char *buf, int *index, int p)

int ei_x_encode_boolean(ei_x_buff* x, int p)

Encodes a boolean value, as the atom true if p is not
zero or false if p is zero.

int ei_encode_char(char *buf, int *index, char p)

int ei_x_encode_char(ei_x_buff* x, char p)

Encodes a char (8-bit) as an integer between 0-255 in the binary format.
Note that for historical reasons the integer argument is of
type char. Your C code should consider the
given argument to be of type unsigned char even if
the C compilers and system may define char to be
signed.

Encodes a string in the binary format. (A string in erlang
is a list, but is encoded as a character array in the binary
format.) The string should be zero-terminated, except for
the ei_x_encode_string_len() function.

int ei_encode_atom(char *buf, int *index, const char *p)

int ei_encode_atom_len(char *buf, int *index, const char *p, int len)

int ei_x_encode_atom(ei_x_buff* x, const char *p)

int ei_x_encode_atom_len(ei_x_buff* x, const char *p, int len)

Encodes an atom in the binary format. The p parameter
is the name of the atom. Only upto MAXATOMLEN bytes
are encoded. The name should be zero-terminated, except for
the ei_x_encode_atom_len() function.

int ei_encode_binary(char *buf, int *index, const void *p, long len)

int ei_x_encode_binary(ei_x_buff* x, const void *p, long len)

Encodes a binary in the binary format. The data is at
p, of len bytes length.

int ei_encode_pid(char *buf, int *index, const erlang_pid *p)

int ei_x_encode_pid(ei_x_buff* x, const erlang_pid *p)

Encodes an erlang process identifier, pid, in the binary
format. The p parameter points to an
erlang_pid structure (which should have been obtained
earlier with ei_decode_pid()).

int ei_encode_fun(char *buf, int *index, const erlang_fun *p)

int ei_x_encode_fun(ei_x_buff* x, const erlang_fun* fun)

Encodes a fun in the binary format. The p parameter
points to an erlang_fun structure. The
erlang_fun is not freed automatically, the
free_fun should be called if the fun is not needed
after encoding.

int ei_encode_port(char *buf, int *index, const erlang_port *p)

int ei_x_encode_port(ei_x_buff* x, const erlang_port *p)

Encodes an erlang port in the binary format. The p
parameter points to a erlang_port structure (which
should have been obtained earlier with
ei_decode_port().

int ei_encode_ref(char *buf, int *index, const erlang_ref *p)

int ei_x_encode_ref(ei_x_buff* x, const erlang_ref *p)

Encodes an erlang reference in the binary format. The
p parameter points to a erlang_ref structure
(which should have been obtained earlier with
ei_decode_ref().

int ei_encode_term(char *buf, int *index, void *t)

int ei_x_encode_term(ei_x_buff* x, void *t)

This function encodes an ETERM, as obtained from
erl_interface. The t parameter is actually an
ETERM pointer. This function doesn't free the
ETERM.

int ei_encode_trace(char *buf, int *index, const erlang_trace *p)

int ei_x_encode_trace(ei_x_buff* x, const erlang_trace *p)

This function encodes an erlang trace token in the binary
format. The p parameter points to a
erlang_trace structure (which should have been
obtained earlier with ei_decode_trace().

int ei_encode_tuple_header(char *buf, int *index, int arity)

int ei_x_encode_tuple_header(ei_x_buff* x, int arity)

This function encodes a tuple header, with a specified
arity. The next arity terms encoded will be the
elements of the tuple. Tuples and lists are encoded
recursively, so that a tuple may contain another tuple or
list.

This function encodes a list header, with a specified
arity. The next arity+1 terms are the elements
(actually its arity cons cells) and the tail of the
list. Lists and tuples are encoded recursively, so that a
list may contain another list or tuple.

Note!

It may seem that there is no way to create a list without
knowing the number of elements in advance. But indeed
there is a way. Note that the list [a, b, c] can be
written as [a | [b | [c]]]. Using this, a list can
be written as conses.

This function returns the type in type and size in
size of the encoded term.
For strings and atoms, size
is the number of characters not including the
terminating 0. For binaries, size is the number of
bytes. For lists and tuples, size is the arity of the
object. For other types, size is 0. In all cases,
index is left unchanged.

int ei_decode_version(const char *buf, int *index, int *version)

This function decodes the version magic number for the
erlang binary term format. It must be the first token in a
binary term.

int ei_decode_long(const char *buf, int *index, long *p)

This function decodes a long integer from the binary format.
Note that if the code is 64 bits the function ei_decode_long() is
exactly the same as ei_decode_longlong().

int ei_decode_ulong(const char *buf, int *index, unsigned long *p)

This function decodes an unsigned long integer from
the binary format.
Note that if the code is 64 bits the function ei_decode_ulong() is
exactly the same as ei_decode_ulonglong().

int ei_decode_longlong(const char *buf, int *index, long long *p)

This function decodes a GCC long long or Visual C++ __int64
(64 bit) integer from the binary format. Note that this
function is missing in the VxWorks port.

This function decodes a GCC unsigned long long or Visual C++
unsigned __int64 (64 bit) integer from the binary format.
Note that this function is missing in the VxWorks port.

int ei_decode_bignum(const char *buf, int *index, mpz_t obj)

This function decodes an integer in the binary format to a GMP mpz_t integer.
To use this function the ei library needs to be configured and compiled
to use the GMP library.

int ei_decode_double(const char *buf, int *index, double *p)

This function decodes an double-precision (64 bit) floating
point number from the binary format.

int ei_decode_boolean(const char *buf, int *index, int *p)

This function decodes a boolean value from the binary
format. A boolean is actually an atom, true decodes 1
and false decodes 0.

int ei_decode_char(const char *buf, int *index, char *p)

This function decodes a char (8-bit) integer between 0-255
from the binary format.
Note that for historical reasons the returned integer is of
type char. Your C code should consider the
returned value to be of type unsigned char even if
the C compilers and system may define char to be
signed.

int ei_decode_string(const char *buf, int *index, char *p)

This function decodes a string from the binary format. A
string in erlang is a list of integers between 0 and
255. Note that since the string is just a list, sometimes
lists are encoded as strings by term_to_binary/1,
even if it was not intended.

The string is copied to p, and enough space must be
allocated. The returned string is null terminated so you
need to add an extra byte to the memory requirement.

int ei_decode_atom(const char *buf, int *index, char *p)

This function decodes an atom from the binary format. The
name of the atom is placed at p. There can be at most
MAXATOMLEN bytes placed in the buffer.

int ei_decode_binary(const char *buf, int *index, void *p, long *len)

This function decodes a binary from the binary format. The
len parameter is set to the actual size of the
binary. Note that ei_decode_binary() assumes that there
are enough room for the binary. The size required can be
fetched by ei_get_type().

int ei_decode_fun(const char *buf, int *index, erlang_fun *p)

void free_fun(erlang_fun* f)

This function decodes a fun from the binary format. The
p parameter should be NULL or point to an
erlang_fun structure. This is the only decode
function that allocates memory; when the erlang_fun
is no longer needed, it should be freed with
free_fun. (This has to do with the arbitrary size of
the environment for a fun.)

int ei_decode_pid(const char *buf, int *index, erlang_pid *p)

Decodes a pid, process identifier, from the binary format.

int ei_decode_port(const char *buf, int *index, erlang_port *p)

This function decodes a port identifier from the binary
format.

int ei_decode_ref(const char *buf, int *index, erlang_ref *p)

This function decodes a reference from the binary format.

int ei_decode_trace(const char *buf, int *index, erlang_trace *p)

Decodes an erlang trace token from the binary format.

int ei_decode_tuple_header(const char *buf, int *index, int *arity)

This function decodes a tuple header, the number of elements
is returned in arity. The tuple elements follows in order in
the buffer.

int ei_decode_list_header(const char *buf, int *index, int *arity)

This function decodes a list header from the binary
format. The number of elements is returned in
arity. The arity+1 elements follows (the last
one is the tail of the list, normally an empty list.) If
arity is 0, it's an empty list.

Note that lists are encoded as strings, if they consist
entirely of integers in the range 0..255. This function will
not decode such strings, use ei_decode_string()
instead.

int ei_decode_ei_term(const char* buf, int* index, ei_term* term)

This function decodes any term, or at least tries to. If the
term pointed at by *index in buf fits in the
term union, it is decoded, and the appropriate field
in term->value is set, and *index is
incremented by the term size.

The function returns 1 on successful decoding, -1 on error,
and 0 if the term seems alright, but does not fit in the
term structure. If it returns 1, the index
will be incremented, and the term contains the
decoded term.

The term structure will contain the arity for a tuple
or list, size for a binary, string or atom. It will contains
a term if it's any of the following: integer, float, atom,
pid, port or ref.

int ei_decode_term(const char *buf, int *index, void *t)

This function decodes a term from the binary format. The
term is return in t as a ETERM*, so t
is actually an ETERM** (see
erl_interface(3). The term should later be
deallocated.

Note that this function is located in the erl_interface
library.

int ei_print_term(FILE* fp, const char* buf, int* index)

int ei_s_print_term(char** s, const char* buf, int* index)

This function prints a term, in clear text, to the file
given by fp, or the buffer pointed to by s. It
tries to resemble the term printing in the erlang shell.

In ei_s_print_term(), the parameter s should
point to a dynamically (malloc) allocated string of
BUFSIZ bytes or a NULL pointer. The string may be
reallocated (and *s may be updated) by this function
if the result is more than BUFSIZ characters. The
string returned is zero-terminated.

The return value is the number of characters written to the
file or string, or -1 if buf[index] doesn't contain a
valid term. Unfortunately, I/O errors on fp is not
checked.

The argument index is updated, i.e. this function can
be viewed as en decode function that decodes a term into a
human readable format.

int ei_x_format(ei_x_buff* x, const char* fmt, ...)

int ei_x_format_wo_ver(ei_x_buff* x, const char *fmt, ... )

Format a term, given as a string, to a buffer. This
functions works like a sprintf for erlang terms. The
fmt contains a format string, with arguments like
~d, to insert terms from variables. The following
formats are supported (with the C types given):

The ei_x_format_wo_ver() formats into a buffer, without
the initial version byte.

int ei_x_new(ei_x_buff* x)

int ei_x_new_with_version(ei_x_buff* x)

This function allocates a new ei_x_buff buffer. The
fields of the structure pointed to by x parameter is
filled in, and a default buffer is allocated. The
ei_x_new_with_version() also puts an initial version
byte, that is used in the binary format. (So that
ei_x_encode_version() won't be needed.)

int ei_x_free(ei_x_buff* x)

This function frees an ei_x_buff buffer. The memory
used by the buffer is returned to the OS.

int ei_x_append(ei_x_buff* x, const ei_x_buff* x2)

int ei_x_append_buf(ei_x_buff* x, const char* buf, int len)

These functions appends data at the end of the buffer x.

int ei_skip_term(const char* buf, int* index)

This function skips a term in the given buffer, it
recursively skips elements of lists and tuples, so that a
full term is skipped. This is a way to get the size of an
erlang term.

buf is the buffer.

index is updated to point right after the term in the
buffer.

Note!

This can be useful when you want to hold arbitrary
terms: just skip them and copy the binary term data to some
buffer.

The function returns 0 on success and -1 on
failure.

Debug Information

Some tips on what to check when the emulator doesn't seem to
receive the terms that you send.

be careful with the version header, use
ei_x_new_with_version() when appropriateturn on distribution tracing on the erlang nodecheck the result codes from ei_decode_-calls